1. Field of the Invention
This invention relates broadly to cutting tools. More particularly, this invention relates to diamond-coated inserts for cutting tools, and a method of manufacturing these diamond-coated inserts.
2. State of the Art
In the machining industry, the type and shape of the cutting point used by a machine to cut a given material is often crucial to its efficiency and accuracy. As illustrated in prior art FIG. 1, this point is usually provided on a detachable piece of material, known as an insert 102, which is made of a very hard material and shaped according to the type of cut desired on the workpiece 101. The cutting tool insert 102 is often attached by a threaded screw 103 to a toolholder 104, which in turn may be fastened to a part of a machine, such as, for example, the carriage 106 of a lathe (not shown). Typically, the insert 102 has multiple corner portions 108a-d for cutting, so that when one corner 108a of the insert 102 is worn, the insert 102 can be repositioned in the toolholder 104 to expose another, unused corner 108b-d. In this manner, a single insert can be repositioned until all of its corners 108a-d are worn. Frequently, an insert has the shape of a triangle or quadrangle (although circular and hexagonal inserts are known) with each corner 108a-d of the insert being used as a cutting point.
Although inserts may be manufactured from a variety of materials, such as metals, carbides and ceramics, for cutting many materials it is preferable to use diamond. With the development of synthetic and thin film diamonds, the use of diamond in cutting tools has become more feasible and prevalent in the cutting tool industry.
Presently, diamond for cutting tools is available in three distinct forms: single crystals; high temperature/high-pressure polycrystalline (PCD) blanks; and, more recently, chemical vapor deposition (CVD) thick-film blanks and thin-film coatings. Due to the different manufacturing processes involved which emphasize certain characteristics at the expense of others, each form is suited to a particular range of applications.
Single crystal inserts are manufactured by shaping natural or artificial diamonds in the form of the cutting sections of an insert, and then brazing the finished diamond onto a substrate. PCD diamond inserts are made by heating and pressurizing a tightly packed mass of diamond particles along with a certain percentage of a sintering aid, typically cobalt. During the sintering process, the cobalt melts and infiltrates the voids between diamond particles. The resulting blank must then be machined to have the desired cutting geometry. CVD diamond inserts are made by either coating a thin film of diamond on a tungsten carbide or ceramic substrate having the desired cutting geometry, or by brazing a free-standing CVD diamond film having the desired cutting shape to the top of the insert where the cutting is to take place. A difficulty common to all three methods of diamond insert manufacture (except where the cut substrate is subjected directly to CVD coating), is that it is often extremely difficult to machine the diamond covered inserts to the desired shape due to the hardness of diamond. Because the inserts must be individually machined, potential inconsistencies in the quality of the inserts are created. As a result, once manufactured, each insert must be quality tested for conformity with other inserts. Furthermore, whenever new insert shapes and sizes are desired, new tools and methods for machining the inserts are required.
As suggested above, in order to substantially eliminate machining, it is known to prepare individual substrates to the desired shape and size and place them in a CVD reactor where the entire tool is subsequently diamond-coated. Consistency, however, is difficult to achieve, as the individually cut substrates may have slightly different geometries. In addition, since the diamond growth rate in a CVD reactor may vary with each use, slightly different insert geometries will result with each use of the CVD reactor. Another potential problem is the handling requirements of present CVD manufacturing methods. As each insert is manufactured and shaped independently, a large number of substrates must be placed in a carefully chosen array, diamond-coated using some appropriate method, inspected, and finally repacked. This entails frequent handling of the inserts. In addition, each different insert geometry requires a specially cut substrate. Inventory requirements and manufacturing time are thereby increased as a result. Also, there is a tendency for diamond coating processes to be sensitive to the size and shapes of each individual insert, thus often requiring manufacturing test runs before an insert with a new size or shape can be made.
In addition to the manufacturing and quality control difficulties in producing the diamond-coated inserts of the prior art, the resulting inserts suffer from certain shortcomings. For example, whenever the cutting point of an insert becomes dull, which may happen frequently depending on the material being worked, the insert must either be discarded or removed from the tool holder and carefully reground. Where only the corners of the insert are diamond-coated, the cutting points may only be reground a limited number of times before the entire insert must be discarded. In the situation where the entire insert substrate is CVD coated with a diamond layer, the diamond layer extends not only along the rake face of the insert but down the flank of the insert. Because of the diamond-coated flank, however, it is extremely difficulty to resharpen the insert. In fact, even if resharpening can be accomplished, the resulting insert will have a different flank configuration than when originally manufactured.